Coal burners with swirl-inducing devices are well known in the art. For example, in U.S. Pat. Nos. 4,249,470 and 4,270,895 to Vatsky, a coal burner is shown and described wherein a mixture of coal powder with steam and/or air is delivered through two concentrically-arranged tubes to a discharge end. A swirler is located at this end and surrounds the tubes to induce swirling motion to secondary combustion air that is delivered through an annular passageway around the outer coal-delivering tube. U.S. Pat. No. 4,690,074 teaches a coal burner wherein the central fuel feed tube is surrounded by a swirling air stream. Another axially-directed air stream surrounds the swirling air stream. The volumes of these latter streams are stated as controllable. U.S. Pat. No. 4,790,743 teaches another coal burner using a swirler that is adjustable. In U.S. Pat. No. 4,569,295 primary air surrounding the fuel supply tube is set in rotation by a swirler formed of guide blades.
Swirl-generating devices are also known to be used in connection with gas burners. The swirlers typically employ vanes in the path of the gaseous fuel to produce a vortex to stabilize the flame position. The vanes are shaped so as to provide less swirl inducement towards the center or roots of the vanes and greater swirl inducement radially outward from the vane roots. As a result, there is more axial fuel flow near the center. However, the gas feed either is located in a small feed tube in the center or spread peripherally around the swirler. The vortex generated by the swirler typically extends to the center axis of the gas burner.
Swirling is controlled with the design or shape of the vanes. A swirl number is used to define critical flow characteristics of a burner. The swirl number is a ratio that defines how much of the combustion air going through a burner is rotating versus how much of the combustion air exiting a burner is in an axial flow condition. Mathematically, swirl number S is defined by the equation: ##EQU1## The numerator represents the product of the radius R and the tangential momentum of the combustion air. The denominator represents the axial momentum of the air and R, is the burner throat radius which varies for the integral from the minimum radius R.sub.hub to the outer periphery R.sub.tip. V.sub.x is the axial velocity; V.sub.t, the tanential velocity and .rho. is the density of the air.
For burners, an important swirl number is 0.6. Swirlers with combustion airflows having swirl numbers less than 0.6 are unstable and exhibit poor fuel/air mixing. An analogy would be the vortex generated when water begins to flow out of a sink. The water begins to rotate and while the water is smooth, but clearly rotating, no depressions form in the surface. Water in this condition has a swirl number less than 0.6. In burner terms, no stabilizing internal recirculation vortices exist.
However, as the water picks up rotational momentum, it begins to develop a depression in the surface. At this point, it crosses the critical boundary of swirl number 0.6, creating an internal recirculating ring vortex. In a burner and its combustion process, the vortex stabilizes the flame in front of the burner.
As the water continues to rotate faster and faster, the depression extends deeper in the water. Eventually, the depression reaches down into the drain, aspirating air along with the water. The swirl number in this last condition is significantly greater than 0.6. While high swirl number combustion is very stable, burner damage can and usually does occur because high temperature furnace gases are aspirated into the burner.
FIG. 1 illustrates the result of so-called burn-back problems frequently encountered with conventional burners having swirl numbers. In FIG. 1, an excessively-swirled burner 8 is shown having fuel supplied through a fuel feed tube 10 to a discharge end 12 located in the throat 14 of the boiler wall 16. A swirler 18 is located in the vicinity of the discharge end 12 and surrounds it in the path of secondary air 20. As a result of the excessive swirl, a rotating hollow cone 22, formed by the combustion air and fuel flowing out of the burner, is resident in the burner. Hot furnace gases move upstream toward the burner along the burner centerline as illustrated by arrows 23.
Eventually, the adverse static pressure of furnace gases moving upstream from the furnace equals the static pressure of the combustion air and fuel flowing out from the burner. The boundary region 24 where this occurs is known as the adverse pressure gradient boundary and represents the boundary at which the furnace gases reverse direction and flow back towards the furnace 26. This reversed flow pattern results in an internal recirculating ring vortex 28 that stabilizes flame fronts. Air, fuel, and high temperature gases are entrained in the ring vortex 28. This then forms a stable ignition zone which remains fixed in position regardless of load as long as the swirl number does not change for the burner.
Single-zone and multi-zoned pulverized coal burners frequently encounter high swirl conditions attributable to swirl design air stream flow variations and under various loading. When excessive swirling occurs the adverse pressure gradient boundary 24 moves upstream as shown in FIG. 1 penetrating burner 8.
Since temperatures above 2500.degree. exist in the flow regime on the furnace side of the adverse pressure gradient boundary 24, burner components which find themselves in this regime experience severe overheating, mechanical distortion, and thermal destruction. Also, fuel entrained in the furnace gases centrifuge out of the flue gas stream at the apex 32 of the adverse pressure gradient boundary 24. The burning fuel deposits in the burner 8 or the windbox 34, accelerating thermal damage. Severe burner destruction problems can occur.
It is, therefore an object of the invention to provide a method for stabilizing a solid fuel burner flame and a swirl flame stabilized solid fuel burner which avoids burn-back problems yet provides a well-stabilized flame throughout burner load variations. It is a further object of the invention to provide a flame stabilizer with which improved solid fuel burner performance can be obtained without flame instability, avoidance of windbox fires, and other burn-back problems. It is still further an object of the invention to provide such flame stabilizer whereby inadequate windbox-to-furnace differential pressure, furnace pulsations, and burner warping can be avoided.